1. Field of the Invention
The present invention relates to a constant-on-time generation circuit, and more particularly, to a constant-on-time generation circuit capable of compensating a turn-on delay time of a high-side switch of a buck converter.
2. Description of the Prior Art
Most electronic products, such as a laptop, a mobile phone, a personal digital assistant, a multimedia player and so on, require a power converter converting an alternate current source into a direct current source to provide a proper input voltage to ensure a normal operation of the electronic products. A buck converter is widely used since it has advantages of simple structure, easy design and low cost.
Please refer to FIG. 1, which is a schematic diagram of a buck converter 10 having a COT (Constant-on-time) control circuit. The buck converter 10 is used for converting an input voltage VIN to an output voltage VOUT to a load RLOAD. The buck converter 10 includes a trigger signal generation circuit 101, a bootstrap circuit 102, an output stage circuit 103, a COT generation circuit 104 and a driver stage circuit 105. The trigger signal generation circuit 101 includes a comparator COMP′ and resistors RF1 and RF2. The bootstrap circuit 102 includes a bootstrap capacitor CBOOT and a diode 107. The output stage circuit 103 includes an output inductor L, an effective serial resistor RESR and an output capacitor COUT. The driver stage circuit 105 includes front-end drivers 106 and 108, a high-side switch HS and a low-side switch LS. Connection relations of above elements are shown in FIG. 1.
In operation, the comparator COMP′ of the trigger signal generation circuit 101 outputs a trigger signal CPOUT to the COT generation circuit 104 according to a trigger reference voltage VREF and a feedback voltage VFB, wherein the output voltage VOUT is divided by the resistors RF1 and RF2 to obtain the feedback voltage VFB denoted as
      V    FB    =                    R                  F          ⁢                                          ⁢          1                                      R                      F            ⁢                                                  ⁢            1                          +                  R                      F            ⁢                                                  ⁢            2                                .  The COT generation circuit 104 generates the turn-on signal STON having a constant turn-on time TON to the front-end driver 106 according to the trigger signal CPOUT, the output voltage VOUT and the input voltage VIN. Moreover, when the high-side switch HS is turned on, a phase signal SUGON is equal to the input voltage VIN, such that the COT generation circuit 104 may generate the turn-on signal STON according to the trigger signal CPOUT, the output voltage VOUT and the phase signal SUGON as well. The front-end drivers 106 and 108 respectively generate a first front-end driver signal UG and a second front-end driver signal LG according to turn-on signal STON and a bootstrap voltage VBOOT and the operating bias VCC to control when to turn on or off the high-side switch HS and the low-side switch LS. For example, the high-side switch HS is turned on and the low-side switch LS is turned off during the turn-on time TON; the high-side switch HS is turned off and the low-side switch LS is turned on during a turn-off time TOFF. The turn-on time TON is predetermined to be a constant, while the turn-off time TOFF is determined according to the feedback voltage VFB, specifically, when the feedback voltage VFB is less than the trigger reference voltage VREF, a switch duty cycle is triggered to start the next turn-on time TON.
However, in the buck converter 10, the high-side switch HS requires a higher drive voltage to be turned on and a longer response time compared to other electronic elements. In other words, when the turn-on STON falls to a low voltage, the high-side switch HS may remain turned on for a while instead of turning off immediately, which causes a turn-on time TON, of the high-side switch HS to be longer than the predetermined turn-on time TON, i.e. TON′=TON+ΔT, wherein ΔT is a delay time of the high-side switch HS.
A duty cycle D of the buck converter is defined to be a ratio of the output voltage VOUT and the input voltage VIN, i.e.
      D    =                  V        OUT                    V        IN              ,furthermore, a switch frequency FSW is defined to be a ratio of the output voltage VOUT and the input voltage VIN per second, i.e.
      F    SW    =                    V        OUT                              V          IN                *                  T          ON                      =                  D                  T          ON                    .      Due to the non-ideal delay time of the high-side switch HS, a real switch frequency FSW′ may be denoted as:
      F    SW    ′    =            D                        T          ON                +                  Δ          ⁢                                          ⁢          T                      .  
Therefore, the delay time ΔT causes the real time TON′ to be different from the default turn-on time, such that the switch frequency FSW′ of the buck converter is changed and does not provide the predetermined or correct switch frequency to the output load. Besides, in practice, the delay time ΔT also causes the switch frequency FSW′ to change with the duty cycle D and a risk of abnormal power supply. Thus, there is a need to improve the prior art.